The present invention relates to an optical super-resolution technique for improving resolution beyond the theoretical resolution limit of an optical device well corrected for aberration. The invention also relates to a technique for eliminating side lobes peculiar to super resolution. More particularly, the invention relates to a technique for improving the resolution of an optical pickup for an optical disk.
The theoretical resolution limit of an optical device will be briefly explained. In an optical device designed to be substantially free from aberration in geometrical optics, the spot image it produces is focused as an infinitely small optical spot. In reality, however, the optical spot exhibits a finite degree of spreading because of the diffraction arising as a consequence of the wave nature of light. Here, when the numerical aperture of the optical device, which contributes to the focusing or converging of the spot, is denoted by NA, the physical definition of the optical spot spreading is given by the formula kxc3x97xcex+NA, where xcex is the wavelength of light, and k is a constant that depends on the optical device and usually takes a value between 1 and 2. The numeral aperture NA is, in general, proportional to D/f which is the ratio of the effective entrance pupil diameter, D, of the optical device (usually, the effective beam diameter) to the focal length, f, of the optical device.
Therefore, if the theoretical resolution of the optical device is to be increased, that is, if the optical spot is to be focused to a smaller diameter, either light of a shorter wavelength should be used or the numerical aperture NA should be increased.
The wavelength of a commonly used laser light source is 780 nm or 650 nm. In recent years, a laser light source with a wavelength of 410 nm has been developed. However, a laser light source having a wavelength of 380 nm or shorter is either difficult to achieve or costly to implement.
On the other hand, as the numerical aperture NA of the optical device increases, it becomes increasingly difficult to design the optical device to be free from aberration in geometric optics. Further, the focal depth of the optical device decreases with the square of the numerical aperture NA, while the coma of the optical device increases with the cube of the numerical aperture NA. Under the current circumstances, therefore, designing an optical device with a numerical aperture NA of about 0.7 or larger is either difficult to achieve or costly to implement.
It should also be noted that optical materials used to construct optical devices are opaque to light at 380 nm and shorter wavelengths. As a result, optical devices using such optical materials have the disadvantage that light cannot be effectively utilized.
Considering the above limitations, the maximum recording density of an optical disk that can be read with the smallest optical spot at the present time is about 12 GB (gigabytes) in the case of an optical disk about 3.5 inches in diameter. Accordingly, if pits exceeding this recording density limit are formed on an optical disk, the pits cannot be read properly with the above optical spot.
In view of this, a technique for achieving a super-resolution optical device, such as described in xe2x80x9cO plus Exe2x80x9d (No. 154, pp. 66-72, 1992), has been proposed in order to further improve the above-described theoretical resolution limit of the optical device. This technique enables the optical spot size to be made 10 to 20% smaller than the theoretical limit of the optical device by blocking a portion of the effective beam of converging optics by means of a light-shielding plate. This is equivalent to increasing the numerical aperture NA of the optical device or making the wavelength of the light source shorter.
The super-resolution optical device, however, has had the problem that when an optical spot is formed, side lobes or relatively large peaks peculiar to super resolution appear on both sides of the spot, making the optical spot appear as if it has three peaks.
This phenomenon will be explained with reference to FIGS. 6 and 7. First, as shown in FIG. 6, the aperture of a converging lens 603 is blocked using a shield mask 602 of radius r around its optical axis 601. The radius r is smaller than the radius of an effective beam 604. FIG. 6 shows a cross-sectional view of the optical device, but it should be noted that the actual optical device has a shape that is rotationally symmetrical about its optical axis 601.
At this time, the optical spot 701 formed at point P, i.e., the focal point of the converging lens 603, can be considered as shown in FIG. 7. That is, the optical spot 701 is the result of subtracting an imaginary optical spot 703, formed due to the shield mask 602, from the optical spot 702 formed by the effective beam 604. It is seen that the optical spot 701 at point P at this time is narrower than the optica spot 702 formed by the effective beam 604, and has side lobes 704 (portions lying in the negative side in FIG. 7).
In FIG. 7, the side lobes 704 are negative in value, and optically this means that the phase of the light wave is shifted by 180 degrees compared with the positive portions, that is, the phase is reversed. From the viewpoint of light intensity, however, these side lobes 704 also have light intensities. As a result, an optical spot having three peaks is formed at point P. In FIG. 7, the complex amplitude is plotted along the vertical axis and the position along the horizontal axis.
The light spot having such three peaks has involved a problem when it is applied, among other things, to optical disk pickups. In view of this, a technique for eliminating only side lobes by placing very fine slits in the light path is proposed in xe2x80x9cOpticsxe2x80x9d (Vol. 18, No. 12, pp. 691-692, 1989). However, the slits have had to be aligned very carefully, since if the slit position is displaced, portions of the optical spot other than the side lobe portions are also blocked. Furthermore, adherence of dust to slit gaps has also been a problem. A further problem has been that since slits are used to block light, even if the slits are properly adjusted in place, diffraction of light still occurs due to the presence of the slits, causing side lobes, though of lesser degree.
Accordingly, it is an object of the present invention to provide an optical device that solves the above problems and that can eliminate only side lobes or side lobe components from a super-resolution optical spot.
It is another object of the invention to provide an optical device that can easily switch between super resolution and normal resolution by using a simple method.
To attain the above objects, the present invention provides the following configuration.
The optical device of the invention is one that includes a light generating means for generating incident light and a lens system for collecting the incident light, and produces a super-resolution optical spot containing a main lobe and a side lobe by modulating a portion of the incident light, and comprises a polarization vector modulating means for making the polarization vectors of the side lobe and the main lobe differ from each other so that one or the other of the polarization vectors can be selected, and a polarization selective means for eliminating the side lobe by selecting the polarization vector of the main lobe.
More particularly, the optical device comprises a means for generating linearly polarized light, an optically rotating element for converting the linearly polarized light into a beam that generates a main lobe and a side lobe oriented in a different direction than the main lobe, and a polarization selective means for eliminating only the side lobe from the beam.
Here, the optically rotating element comprises a homogeneous-type liquid crystal element and a 90-degree twisted nematic liquid crystal element whose orientation axis of liquid-crystal molecular is oriented substantially parallel or perpendicular to the polarization axis of the linearly polarized light.
Further, the polarization selective means is disposed so as to have as azimuth whose angle, relative to the azimuth of the linearly polarized light incident on the optically rotating element, is not smaller than 0 degree and not greater than 90 degrees, when measured toward the direction in which the optically rotating element rotates the linearly polarized light through 90 degrees.
In another aspect of the invention, the optical device comprises a means for generating linearly polarized light, an optically rotating element for converting the linearly polarized light into a beam that generates a main lobe and a side lobe oriented in a direction different to the main lobe, a first collective lens for collecting the beam onto an optical disk; an optical detector for detecting information recorded on the optical disk, a second collective lens for collecting a beam, reflected from the optical disk, onto the optical detector; and a polarization selective means for eliminating only the side lobe from the beam.
Here, the optically rotating element has a rotatory power that is capable of being enabled or disabled electrically, and the beam is converged on a different kind of optical disk when the rotatory power of the optically rotating element is disabled than when the optical activity of the optically rotating element is enabled.
The different kinds of optical disks here refer, for example, to a DVD and a CD, respectively, or a DVD and a CD-R(W), respectively.
In a further aspect of the invention, the optical device comprises a means for generating linearly polarized light, a diffraction lens element whose diffraction function is capable of being enabled or disabled by an electrical signal, an optically rotating element for converting the linearly polarized light into a beam that generates a main lobe and a side lobe oriented in a different direction than the main lobe, a collective lens for collecting the beam onto an optical disk, and a polarization selective means for eliminating only the side lobe from the beam.
Here, when the diffraction function of the diffraction lens element is enabled, the optical device has a focal length equal to the sum of the focal lengths of the diffraction lens element and the collective lens, while when the diffraction function of the diffraction lens element is disabled, the focal length of the optical device is equal to the focal length of only the converging lens.
Further, the beam is converged on a different kind of optical disk when the diffraction function of the diffraction lens element is disabled than when the diffraction function of the diffraction lens element is enabled.
The different kinds of optical disks here refer, for example, to a DVD and a CD, respectively, or a DVD and a CD-R(W), respectively.